libpng16: Update version from 1.6.34 to 1.6.37

From the [Web site][1]:

> libpng versions 1.6.36 and earlier have a use-after-free bug in the
> simplified libpng API png_image_free(). It has been assigned ID
> CVE-2019-7317. The vulnerability is fixed in version 1.6.37, released
> on 15 April 2019.

[1]: http://www.libpng.org/pub/png/libpng.html

libpng16.be0

@@ -1,6 +1,6 @@
 #!/usr/bin/env beesh
 
-# BEE_VERSION libpng16-1.6.34-0
+# BEE_VERSION libpng16-1.6.37-0
 
 SRCURL[0]="https://downloads.sourceforge.net/libpng/libpng-${PKGVERSION}.tar.xz"